Various types of microphones and receivers have been used through the years. In these devices, different electrical components are housed together within a housing or assembly. For example, a receiver typically includes a coil, bobbin, stack, among other components and these components are housed within the receiver housing. Other types of acoustic devices may include other types of components. A twin receiver design can be used, one receiver providing for the outputs in the “tweeter” sound range and the other for providing outputs in the “woofer” sound range.
Various two-way receivers are used in earphones and these utilize a single capacitor to shape the high frequency driver (i.e., tweeter) response and no filtering on the low frequency driver (i.e., woofer) response. Sometimes, the natural low-pass shape of the woofer is effective to keep it from overlapping the response of the tweeter receiver. However, this performance limits the cross-over frequency to be a high frequency, which is typically between 2 and 3 kHz. This produces undesirable results in many systems.
Previous attempts at solving this problem have changed the mass and/or stiffness of the motor/diaphragm of the receivers. However, adding mass to the system can have undesirable side effects. For instance, the high mass may cause an uneven acoustic response and make the unit easily damaged when dropped. The uneven response leads to poor sound quality for the listener.
An electric low pass filter can also be added to the system. Inductors can be used to create the low pass filter. However, one problem of this attempted solution is that if inductors are used, they must be large—in some cases larger than the receiver itself. Since the inductors are so large, the resulting device is too large or cumbersome for practical use in many applications. An acoustic low pass filter can be created by adding a long thin tube to the output of the receiver, or by using one or more very small openings in the receiver outlet. Such a filter will reduce the high frequencies, but will still have an undesirable resonance in the 3-5 kHz region. The volume of air trapped between the diaphragm and the receiver outlet forms a compliance, which interacts with the mass of the moving parts in the receiver to form a resonance.
Because of these shortcomings, previous approaches have not adequately addressed the above-mentioned problems and user dissatisfaction with these previous approaches has increased.
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